Development of Transfer jet has advanced as a proximity wireless communication system between two communication devices which are in proximity to each other to form a gap of approximately several cm therebetween.
To perform communication by utilizing this type of proximity wireless communication system, coupler apparatuses having two communication devices mounted thereon, respectively, are proximally positioned to face each other. Each coupler apparatus includes a coupling element and utilizes electromagnetic coupling between the coupling elements to transmit or receive electromagnetic waves.
A coupler apparatus is generally constituted by arranging a coupling element and a ground plane, each of which is obtained by forming a conductive material into a tabular shape, to face each other. Further, in the coupler apparatus on a transmission side, an electromagnetic field is generated around the coupler apparatus by feeding a signal to a portion between the coupling element and the ground plane to generate an electric current in the coupling element, thereby producing electromagnetic coupling between this coupler apparatus and a coupler apparatus on a reception side. In the coupler apparatus on the reception side, the above-described signal can be fetched in accordance with a potential difference between the coupling element and the ground plane when the electric current is generated in the coupling element based on the produced electromagnetic coupling.
In this type of coupler apparatus, electromagnetic coupling concerning a necessary frequency band is generated by an electric current produced between a feeding point and an open end.
On the other hand, a short element may be arranged between the coupling element and the ground plane to short-circuit the coupling element and the ground plane. In this case, the short element is provided at one position avoiding a current path between the feeding point and the open end.
Meanwhile, in the coupler apparatus having the short element provided therein, an electric current flowing on the coupling element between the feeding point and the short element is also generated. Further, this current becomes larger than the current between the feeding point and the open end, and it is generated while being biased toward one direction from the feeding point, whereby a current distribution of the entire coupler apparatus is biased. Furthermore, such bias of the current distribution may possibly lead to a reduction in a degree of coupling with another coupler apparatus.
It is to be noted that JP-A 2006-197449 (KOKAI) discloses an antenna apparatus having two short-circuit pins provided between a top plate conductor and a ground plane conductor.
However, in the antenna apparatus disclosed in the above-described document, the short-short pins are arranged in the middle of a path for a current flowing through the top plate conductor. That is because the antenna apparatus disclosed in the above-described document is configured to be omnidirectional on a plane where the top plate conductor is present and to have characteristics of avoiding emission of an electric wave in a direction orthogonal to the top plate conductor and this apparatus utilizes an electromagnetic function absolutely different from the coupler apparatus that produces electromagnetic coupling in a direction orthogonal to the coupling element.
Under such circumstances, the coupler apparatus has been demanded to enable suppressing a change in transmission coefficient involved by rotation around central axis even in a state that it faces another coupler apparatus with their central axes deviating from each other.